APOBEC3G (A3G) is a cellular antiviral protein that, when incorporated into a budding HIV-1 virion, dramatically reduces infectivity in the subsequent cell (see, e.g., Harris et al. (2004) Nat Rev Immunol 4, 868-877; Navarro et al. (2004) Curr Opin Immunol 16, 477-482; Turelli et al. (2005) Science 307, 1061-1065). However, A3G incorporation into the virion is dramatically reduced due to the action of the HIV-1 Vif protein. HIV-1 Vif hijacks the cellular ubiquitin proteasome system and targets A3G for proteasomal degradation (see, e.g., Yu et al. (2003) Science 302, 1056-1060; Marin et al. (2003) Nat Med 9, 1398-1403; Liu et al. (2005) J Virol 79, 9579-9587; Kobayashi et al. (2005) J Biol Chem 280, 18573-18578).
The Cullin E3 ubiquitin ligases are a family of modular RING E3 ligases that consist of three main components: a Cullin (Cul1,2,3,4a,4b,5, or 7), an adaptor protein, and a substrate receptor. The E3 ligase is the third enzyme in the ubiquitination sequence and is responsible for substrate specificity. Cullin acts as scaffolding upon which the adaptor protein and substrate receptor assemble in order to bring a specific substrate in close proximity to the E2 ubiquitin conjugating enzyme. The substrate receptor determines the specificity of the protein to be degraded and binds to Cullin via an adaptor protein. The E2 conjugating enzyme transfers multiple ubiquitin molecules to the substrate, targeting it for proteasomal degradation. HIV-1 Vif co-opts the Cullin5 E3 ubiquitin ligase, acting as a substrate receptor, targeting A3G for proteasomal degradation (see FIG. 1A). Both Cul2 and Cul5 bind their substrate receptors through the ElonginB-ElonginC adaptor proteins. Cellular substrate receptors have an additional interface that determines Cul2 or Cul5 selection, termed the Cul2 or Cul5 box, respectively.
Primate lentiviral Vif proteins do not have a Cul5 box although they specifically select Cul5. Some of the inventors and colleagues have previously identified a highly conserved HCCH zinc binding motif and demonstrated its requirement for Cul5 selection (see, e.g., Yu et al. (2004) Genes Dev 18, 2867-2872; Luo et al. (2005) Proc Natl Acad Sci USA 102, 11444-11449; Xiao et al. Virology 2006 Jun. 5; 349(2):290-9. Epub 2006 Mar. 13). Without wishing to be bound by any particular mechanism, it is suggested that this zinc binding domain acts to stabilize a putative helix with a hydrophobic face that is required for Cul5 interaction (see FIG. 1B).
It would be desirable to identify an agent (e.g., a therapeutic agent) that can, e.g., inhibit HIV-1 Vif mediated A3G degradation in a cell, thereby blocking the degradation of the antiviral agent, A3G, and thus inhibiting HIV-1 infectivity.